Conventional ultrasound imaging systems comprise an array of ultrasonic transducers which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. For ultrasound imaging, the array typically has a multiplicity of transducers arranged in a line and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducers can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer is employed to receive the reflected sound (receiver mode). The voltages produced at the receiving transducers are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delays (and/or phase shifts) and gains to the signal from each receiving transducer.
FIG. 1 depicts a conventional B-mode ultrasound imaging system comprising a beamformer 2, a B-mode processor 4, a scan converter/display controller 6 and a kernel 8. System control is centered in the kernel, which accepts operator inputs through an operator interface 10 and in turn controls the various subsystems. The master controller 12 performs system level control functions. It accepts inputs from the operator via the operator interface 10 as well as system status changes and makes appropriate system changes either directly or via the scan controller. The system control bus 14 provides the interface from the master controller to the subsystems. The scan control sequencer 16 provides real-time (acoustic vector rate) control inputs to the beamformer 2, system timing generator 24, B-mode processor 4 and scan converter 6. A scan control sequencer 16 is programmed by the host with the vector sequences and synchronization options for acoustic frame acquisitions. The scan control sequencer broadcasts the vector parameters defined by the host to the subsystems via scan control bus 18.
The main data path begins with the analog RF inputs to the beamformer 2 from a transducer array 20. The beamformer 2 converts the analog signal into a stream of digital samples and outputs two summed digital receive beams, which are shown as complex I,Q data in FIG. 1, but in general, can also be radio or intermediate frequency data. The I,Q data is input to B-mode processor 4 and output as processed vector (beam) data to the scan converter/display processor 6. The scan converter accepts the processed vector data and outputs the video display signals for the image to a color monitor 22.
Referring to FIG. 2, a conventional ultrasonic imaging system incorporates a transducer array 20 comprised of a plurality of separately driven transducer elements 26, each of which produces a burst of ultrasonic energy when energized by a pulsed waveform produced by a transmit section of the beamformer 2. The ultrasonic energy reflected back to transducer array 20 from the object under study is converted to an electrical signal by each receiving transducer element 26 and applied separately to a receive section of beamformer 2 through a set of transmit/receive (T/R) switches 28. The T/R switches 28 are typically diodes which protect the receive electronics from the high voltages generated by the transmit electronics. The transmit signal causes the diodes to shut off or limit the signal to the receive section.
The transmit and receive sections of the beamformer are operated under control of a beamformer controller (not shown) responsive to commands by a human operator. The elements of the transducer array 20 are driven such that the ultrasonic energy produced is directed, or steered, in a beam. To accomplish this, respective time delays are imparted to a multiplicity of pulsers 28. Each pulser is connected to a respective transducer element via the T/R switches. The transmit focus time delays are preferably read from a look-up table 32. By appropriately adjusting the transmit focus time delays in a conventional manner, the ultrasonic beam can be directed away from axis Y by an angle .theta. and/or focused at a fixed range R. A sector scan is performed by progressively changing the transmit focus time delays in successive excitations. The angle .theta. is thus changed in increments to steer the transmitted beam in a succession of directions.
The echo signals produced by each burst of ultrasonic energy reflect from objects located at successive ranges along the ultrasonic beam. The echo signals are sensed separately by each transducer element 26 and a sample of the magnitude of the echo signal at a particular point in time represents the amount of reflection occurring at a specific range. Due to the differences in the propagation paths between a reflecting point P and each transducer element 26, however, these echo signals will not be detected simultaneously and their amplitudes will not be equal. The receive section of beamformer 2 imparts the proper time delay to each received signal and sums them to provide a single echo signal which accurately indicates the total ultrasonic energy reflected from point P located at range R along the ultrasonic beam oriented at the angle .theta.. To accomplish this, respective receive focus time delays are imparted to a multiplicity of receive channels 34. Each receive channel is connected to a respective transducer element via a T/R switch. The receive focus time delays are preferably read from a look-up table 38. The receive channels also have circuitry (not shown) for apodizing and filtering the received pulses. The time-delayed receive signals are then summed in receive summer 36.
Referring to FIG. 3, the receive section of beamformer 2 comprises a time-gain control section and a receive beamforming section. Time-gain control (TGC) section includes a respective amplifier 40 for each of the receiver channels 34 and a time-gain control circuit 42. The input of each amplifier 40 is connected to a respective transducer element 26 to amplify the echo signal which it receives. The amount of amplification provided by amplifiers 40 is controlled by a TGC circuit 44, which is set by hand operation of a multiplicity of potentiometers 44.
The receive beamforming section comprises separate receive channels 34. Each receive channel 34 receives the analog echo signal from one of amplifiers 40. Each amplified signal is conveyed to a pair of quadrature detectors in the respective receiver channel, where the phases of the mixing reference frequency differ by 90.degree.. Since this reference frequency is made identical to that of the transmitted pulses, the outputs from a low-pass filter in the receive channel become complex signals (I and Q) with phases that differ by 90.degree.. These signals are output as a stream of digitized output values on an I bus 45a and a Q bus 45b (or an equivalent RF bus). Each of these I and Q baseband signals represents a demodulated sample of the echo signal envelope at a specific range R. These samples have been delayed such that when they are summed at summing points 36a and 36b with the I and Q samples from each of the other receive channels 34, the summed signals indicate the magnitude and phase of the echo signal reflected from a point P located at range R on the steered beam (.theta.).
A detector 46, incorporated in the B-mode processor 4, receives the beam samples from summing points 45a and 45b. The I and Q values of each beam sample are signals representing the in-phase and quadrature components of the magnitude of the reflected sound from a point (R, .theta.). The detector 46 computes the quantity (I.sup.2 +Q.sup.2).sup.1/2, which represents the envelope of the baseband data. If the beam samples are RF data, the envelope of the signal can be obtained via a standard rectifier followed by a low pass filter. The B-mode function images the time-varying amplitude of the envelope of the signal as a grey scale with some additional processing (hereinafter referred to as "post-detection image processing"), such as edge enhancement and logarithmic compression.
The scan converter 64 (see FIG. 1) receives the display data from the B-mode processor 4 and converts the data into the desired image for display. In particular, the scan converter 24 converts the acoustic image data from polar coordinate (R-.theta.) sector format or Cartesian coordinate linear array to appropriately scaled Cartesian coordinate display pixel data at the video rate. This scan-converted acoustic data is then output for display on display monitor 22, which images the time-varying amplitude of the envelope of the signal as a grey scale.
Ultrasound imaging suffers from the inherent imaging artifact referred to as speckle. Speckle is the mottling found in the images produced from interference patterns of multiple receive echoes. This mottling is primarily caused by the null in the acoustic interference pattern, but other anomalies in the image, e.g., random electronic noise, can cause mottling. The acoustic nulls are accentuated by the logarithmic compression required to display the full dynamic range of the ultrasound image. These nulls appear as black holes in the image. It is desirable to minimize speckle to improve image quality.
Post-detection image processing generally consists of dynamic range (logarithmic) compression, a low pass filter and an edge enhancement filter. While these may be arranged in different orders on different scanners, they are usually done in a sequential manner. Traditionally the low pass filter is designed to prevent aliasing before data downsampling, but it can also play the role of speckle reduction in a wideband imaging system. The edge enhancement filter is usually a high pass filter that acts on the log-compressed data.
Detected images of real anatomy usually contain both large reflected signals (from edges) and low amplitude speckle (from soft tissue). Therefore, if the low pass filter and high pass filter simply perform their tasks blindly, there is always a tendency for these filters to smear out edges and enhance background speckle respectively. With a sequential processing configuration, it is very difficult to keep the low pass and high pass filters from acting on the wrong features. The best that can often be achieved is a compromise between speckle smoothing and edge enhancement.
FIGS. 4A-4C shows three basic prior art configurations for sequential post-detection processing in an ultrasound imaging system. The first configuration shown in FIG. 4A is similar to the post-detection processing method used on analog systems and comprises in sequence means for logarithmic data compression 48, an edge-enhancing high pass filter 50 and a low pass filter 52. The low pass filter 52 is typically a 4th to 6th order IIR filter with a frequency cutoff set according to the decimation rate before scan conversion. An advantage of the first configuration is that the high pass filter 50 is effective for enhancing edges in log-compressed images. If the detected image is subject to high pass filtering before logarithmic compression, any edge enhancement effect would likely be diminished by the logarithmic compression. One disadvantage of the first configuration is that a post-logarithmic compression low pass filter is intended for anti-aliasing and is less effective for reducing speckle.
A second configuration is shown in FIG. 4B. In an actual implementation of this configuration, the decimator or rate converter may appear before the edge enhancement filter 50. Thus, the low pass filter 52 can serve the dual purpose of speckle smoothing and anti-aliasing. One advantage of the second configuration is that the low pass filter is optimally positioned (upstream of the nonlinear compression) for speckle reduction. Disadvantages of the second configuration are that the low pass filter tends to smear out edges; the high pass filter may also enhance the background speckle, thereby canceling out the effects of the low pass filter; and the low pass filter position is not optimal for anti-aliasing since the signal bandwidth expands again after logarithmic compression.
A third configuration is shown in FIG. 4C. As in the second configuration, decimation and rate conversion can occur before edge enhancement filtering. The main improvement lies in the adaptive nature of the edge enhancement filter, which is typically comprised of parallel high pass filter and all-pass paths. Advantages of the third configuration are that the adaptive edge enhancement filter tries to differentiate edges from speckle based on their amplitude differences; and that the edge enhancement filter tends to enhance large amplitude edges only. Disadvantages include the following: the edges are already smeared by the low pass filter before the processed signals reach the adaptive edge enhancement filter; and the position of the adaptive mechanism is not optimal, i.e., differences between edge and speckle amplitudes have already been significantly reduced by the logarithmic compression.
In accordance with a fourth configuration (not shown), the RF spectrum is split into two or more sub-bands which are then detected separately. Summing the incoherent images after detection is an effective way of reducing speckle. However, this form of frequency compounding has the same statistical performance as that of the second configuration shown in FIG. 4B. While the incoherent summing after detection can reduce speckle, the splitting of the RF spectrum has the same resolution degradation (edge smearing) effect as the low pass filter in the second configuration.